cd pushes to directory stack

Created by: zen3ger

Reproduction:

zen3ger:~# dirs -v
 0  /home/zen3ger
zen3ger:~# cd Projects/
zen3ger:~/Projects# dirs -v
 0  /home/zen3ger/Projects
 1  /home/zen3ger
zen3ger:~/Projects# cd ../../../
zen3ger:/# dirs -v
 0  /
 1  /home/zen3ger/Projects
 2  /home/zen3ger

Expected behavior: cd operates on the same directory stack.

Actual behavior: cd always pushes path to a new directory stack.

Build information: ion 1.0.0-alpha (x86_64-unknown-linux-gnu) rev 18b6f5eb